1. Field of the Invention
This invention relates to stereo audio apparatus having two or more audio output channels.
2. Prior Art
Multi-channel sound reproducing apparatus may, for the present purposes, be considered as being divided into two distinct types namely (1) systems where the noise has a similar value on both channels and (2) systems where the noise on one channel is considerably different from that on the other channel. The present invention is applicable to both these types but, as will be apparent from the following description has particular advantages with the second type where, for reasons discussed below, the problem of noise is much more serious.
In stereo sound radio transmissions, the present practice is to transmit, as a frequency modulation on the radio frequency carrier, a composite signal which, when demodulated has signals in the band from 0 to 15 kHz which are referred to hereinafter as the A+B signals and which correspond to the sum of the sound signals from the left and right audio transducers and also has, centred on a 38 kHz sub-carrier, signals within the band from about 23 kHz to 53 kHz which are referred to hereinafter as the A-B signals and which correspond to the difference of the required outputs from the left and right-hand audio transducers. These two sets of signals are separated in the receiver and then fed to a decoding matrix which combines the signals so as to provide separate A and B outputs, that is to say separate outputs which may be fed to the left and right-hand audio transducers. The above-described system for transmitting stereo programmes on radio provides a compatible system in that such programmes can be received on a "mono" receiver, that is to say a receiver responsive to demodulated signals in the audio band and having a single sound transducer. The output from the sound transducer is the A+B signal.
However, as is well known, the reception of stereo programmes on radio is often very poor, particularly for weak signals, and is much degraded compared with reception in the mono form. This degraded performance is inherent in the use of frequency modulation and the method of stereo coding which gives a noise factor on stereo which is much worse than on mono reception. The noise factor is typically 10 times worse on stereo. Thus for weak stereo signals or poor tuners giving only just adequate mono performance, this means that the programme can be tolerated in the mono mode but not in the stereo mode because of the increased noise generated by the stereo decoder. In addition, the stereo noise produced is weighted towards the high frequency part of the audio spectrum. The human ear is subjectively far more sensitive to random noise at the high frequency part of the audio spectrum compared with the lower frequency end where the ear is relatively tolerant to quite large amounts of noise.
The present invention is directed to reducing the noise level in the audio frequency output of a stereo audio apparatus. It is applicable both to systems where the noise level is similar on various channels and to systems where there are different noise levels on the various channels.
Heretofore, the problem of noise in stereo broadcast systems has been tackled by trying to reduce the band width by using amplitude compression before transmitting the signals and providing complementary expansion in the receiver. To introduce such techniques however requires modification of both transmitting and receiving equipment and cannot therefore readily be introduced into existing equipment.